as pointed by WiiPower (the creator of NeoGamma), Neogamma is not compatible with rev15. Probably what you are getting is the Error 001, the one that is occuring with this combination. If everything was running smoothly with rev 14, then reinstall it and follow thes einstructions (taken from the NSMB Wii all infos topic)

Quoted from QUOTE:

For Disc-Loader Users (no ModChip, only cIOS):
-The patched game with the PPF-Files below works fine with rev14 and NeoGamma/SoftChip.
-The patched game works with cIOS rev15, and USBLoader GX or uLoader 3.1.

QUOTEPatching/Fixing the ISO:

1) Download the WiiScrubber 1.40 Package
2) Run the "MakeKeyBin.exe"
3) Open WiiScrubber, and select your ISO
4) Search for the main.dol -> right click -> Extract
5) Download THIS Package here, and your favourite PPF-Patcher (ex. PPF-O-Matic)
6) Open your PPF-Patcher, choose the main.dol for input and the PPFf-Patch for your region, and klick apply.
This is the patched main.dol that you can use for alt.dol-Loading. If you want to patch the ISO, read further.
7) Back in WiiScrubber, search again for the main.dol, right klick -> Replace and select the patched main.dol
8) Done. Exit WiiScrubber, and enjoy!

Or you can search for on the know ISO Sites for an already fixed release. DON'T ASK ANYONE WHERE TO GET THESE!!!